1. Field of the Invention
The present invention relates to monolithic ceramic electronic components, and more particularly, the present invention relates to inductors, LC combined electronic components, LR combined electronic components, and LCR combined electronic components, which can operate at high frequencies.
2. Description of the Related Art
In recent years, electronic devices operating at high frequencies have become common. Inductors, LC combined components, LR combined components, LCR combined components, and the like, which can operate at gigahertz (GHz) frequencies, have become necessary.
However, in an inductor for high-frequency operation, stray capacitance occurring in parallel with the inductor seriously affects the impedance of the inductor. In particular, at GHz frequencies, small stray capacitance, in the range of 0.01 pF to 0.1 pF, seriously affects the impedance. Consequently, to achieve the desired characteristics by decreasing the stray capacitance, it is necessary to decrease the dielectric constant ε of ferrite for a magnetic material. Unfortunately, decreasing the dielectric constant ε of ferrite, for example, down to 14 or less, is difficult practically because of the structure of ferrite.
Thus, a method for decreasing the dielectric constant by mixing a magnetic material with a material such as a resin and glass having a low dielectric constant is suggested. In such a magnetic composite that is composed of a magnetic material and a non-magnetic material such as a resin and glass, the particles of the magnetic material are covered with the non-magnetic material to interrupt a magnetic path. As a result, permeability is decreased dramatically.
Japanese Unexamined Patent Application No. 55-52300 discloses porous sintered ferrite having a porosity of 20% to 70% for an electromagnetic wave absorber, the porous sintered ferrite having a low dielectric constant because of its high porosity. Japanese Unexamined Patent Application No. 11-67575 discloses a ceramic electronic component provided with ceramic and an inner electrode disposed within the ceramic, the ceramic having pores with a diameter of 1 μm to 3 μm and having a porosity of 3 to 30 percent by volume.
Such a porous sintered ferrite has a low dielectric constant because of its high porosity, thus improving impedance characteristics at high frequencies. In addition, since such a porous sintered ferrite has continuous magnetic paths, the electromagnetic properties of the porous sintered ferrite do not vary significantly.
However, in a typical chip inductor composed of a non-porous ceramic body, superimposing a direct current impairs the impedance characteristics at lower frequencies than its self-resonant frequency and causes a change in the self-resonant frequency. Hence, even if the self-resonant frequency without the superimposed direct current is adjusted to noise frequencies, noise cannot be effectively removed because of the change in the self-resonant frequency. FIG. 4 shows that the rate of change in impedance at 100 MHz with a superimposed direct current of 100 mA relative to that without a superimposed direct current is −60.9%.
On the other hand, in a chip inductor composed of a porous ceramic body, the self-resonant frequency does not change by superimposing a direct current, but the impedance is significantly decreased. FIG. 5 shows that the rate of change in the impedance at 100 MHz with the superimposed direct current of 100 mA relative to that without the superimposed direct current is −57.4%.